As various mobile communication systems are developed, an efficiency of mobile terminals becomes a very important factor. Specifically, low-power circuits for the mobile terminals are required because the capacities and the sizes of battery cells have a limitation. Among components of the mobile terminals, power amplifiers are dominant power-consumers, so that the efficiency of the mobile terminals adopting, e.g., a CDMA or an OFDM, is at most 20%. The reason why the efficiency of the mobile terminals adopting the CDMA or the OFDM is low is that the envelopes of signals vary. For the purpose of amplifying the signals whose envelopes vary in the time domain, linear amplifiers of, e.g., class A or class AB are required. However, the class A or the class AB amplifier has poor efficiency in that they have great power losses.
Various techniques have been proposed for amplifying a signal linearly with much better efficiency. Among the techniques is Pulse Width Modulation (i.e., PWM) capable of amplifying a signal whose envelope varies in the time domain. A high-efficiency linear power amplifier system adopting the PWM was suggested by Yuanxun Wang (see “An Improved Kahn Transmitter Architecture Based on Delta-Sigma Modulation”, 2003 IEEE MTT-S Int. Microwave Symp. Dig., pp. 1327-1330). Specifically, a radio frequency (RF) signal fed into the high-efficiency linear power amplifier system is modulated by using the PWM; the pulse-width modulated signal is amplified by applying it to a switching amplifier such as class D, E or F power amplifier; and the amplified signal, i.e., the output of the switching amplifier, passes through a band pass filter (i.e., BPF), thereby restoring the waveform of the original RF signal. Herein, the linear power amplifier system has high efficiency in that the switching amplifier is operated only under two modes, i.e., ON and OFF, and has good linearity in that the amplitude of the output thereof is proportional to only the pulse width of the signal modulated by the PWM.
FIG. 1 shows a block diagram of a conventional high-efficiency linear power amplifier system using the pulse width modulation (PWM). The conventional high-efficiency linear power amplifier system includes an envelope/phase decomposer 102, a pulse width modulator 103, a mixer 104, a high-efficiency power amplifier 105 and a band pass filter (BPF) 106. Specifically, an RF input signal 101 is decomposed into an envelope signal 108 and an RF phase signal 109 by the envelope/phase decomposer 102, wherein the RF phase signal 109, whose amplitude does not vary in the time domain, includes only phase information of the RF input signal 101. Then, the pulse width modulator 103 modulates the decomposed envelope signal 108 to thereby produce a pulse-width modulated signal whose pulse width is proportional to the amplitude of the envelope signal 108. On the other hand, the decomposed phase signal 109 is mixed with the pulse-width modulated signal at the mixer 104 to thereby generate an RF pulse train 110, which is fed into the high-efficiency power amplifier 105, e.g., a switching power amplifier such as class D, E or F power amplifier. Subsequently, the high-efficiency power amplifier 105 amplifies the RF pulse train 110 to thereby create an amplified RF pulse train, which is applied to the BPF 106. Finally, after filtering harmonic components of the amplified RF pulse train at the BPF 106, an amplified input signal 107 whose waveform is identical to the original RF input signal 101 except the size thereof may be restored.
Although the high-efficiency linear power amplifier system has ideally 100% efficiency, the BPF 106 may not eliminate the harmonic components perfectly because the harmonic components are generated adjacent to an inband signal in the frequency domain, resulting in degradation of the linearity, i.e., the efficiency of the linear power amplifier.
FIG. 2 is a table showing the variance of amounts of both the inband component and the harmonic components as a function of a pulse width, which is proportional to an amplitude of a baseband signal, wherein the baseband signal is modulated by the PWM. Referring to FIG. 2, as the amplitude of the baseband signal decreases, i.e., as the pulse width decreases, the amount of the harmonic components becomes relatively larger than that of the inband component.